Use a regular circular saw with a diamond blade.
Keep it wet while cutting - I use a small pump type weed sprayer when using my saw to cut concrete and the like...
Cut slowly and should work well....
Makae sure you are using a grounded cord and plugged into a GFCI splitter or outlet for safety...

I got two custom-molded vanity tops of odd width and lengths for a very reasonable price. They were 43" long, about 19.8" deep to fit a 40 year old custom installation, and they even accommodated the slightly out-of-square of the walls. They had the recessed bowls like you described. Cost for each was $215 in 2004.

They were a faux marble from a resin material with the typical color variation. They are more rugged than the cultured marble that I saw, which reminded me of porous plaster with surface finish.

When you go to cut, you may want to have a pattern from plywood or masonite. It is very hard to measure and then lay out to fit a non-square configuration.

With a 58" long piece you will want to be very careful about straightness of the back wall. If you have 1/4" of bow in the wall, and a molded-in backsplash, the variation in spacing from the wall or variation of thickness of the backsplash will be quite apparent.

With a 58" long piece you will want to be very careful about straightness of the back wall. If you have 1/4" of bow in the wall, and a molded-in backsplash, the variation in spacing from the wall or variation of thickness of the backsplash will be quite apparent.